Light-colored rocks scattered across Mars have become a quiet clue to a louder story about the planet\u2019s past. NASA\u2019s Perseverance rover keeps finding pale pebbles, cobbles and even small boulders lying loose on the floor of Jezero crater and across its ancient river delta. They look out of place against the darker bedrock. They are not attached to anything, which is why scientists call them \u201cfloat rocks.\u201d Their chemistry suggests they traveled from elsewhere and landed here long ago.<\/p>\n
As the rover zapped and scanned these fragments, researchers noticed something odd. Many are packed with aluminum and carry the distinct signal of kaolin-group minerals such as kaolinite and halloysite. On Earth, those minerals typically form where liquid water<\/a> weathers rock for long periods. That discovery raised a simple but powerful question: are these pieces of ancient Martian soils?<\/p>\n So far, about 20 of the pale rocks have been probed with SuperCam and Mastcam-Z. Some contain between 17 and 45 percent aluminum oxide and very little iron. Others are rich in silica, with more than 90 percent silicon dioxide. A few even hold nickel at levels that match ore bodies on Earth. Combined, the results point to rocks that were profoundly altered by water before they broke apart and later reached Jezero.<\/p>\n Purdue University research into scattered kaolinite rocks on Mars\u2019 surface shows the dry, dusty planet could have featured a rain-heavy climate billions of years ago. (CREDIT: NASA) A trail of pale stones<\/p>\n Perseverance first found these rocks on the crater floor. Then they appeared again on the front of the fan where a river once poured into a lake. They also dot the top of that fan. Sizes vary from pebbles you could pocket to boulders that would challenge two people to lift. Their colors range from bone white to dusty gray and tan.<\/p>\n One cluster stood out when the rover reached it around mission day 912. There, dozens of fragments lay strewn across an area the size of a small room. Multispectral images revealed that many glowed differently in near-infrared light, hinting at stark differences from the surrounding terrain and from each other.<\/p>\n Within this mix, a \u201chydrated\u201d group grabbed attention. Targets called Chignik, Rainbow Curve, Elk Mountain and Coral Bay show signs of water locked inside their minerals. Their spectra include a mark near 2.21 micrometers from aluminum linked to hydroxyl groups, and another near 1.9 micrometers from molecular water. In simpler terms, these rocks still hold evidence of moisture<\/a> at the atomic level.<\/p>\n Other fragments look different. Some appear dehydrated and contain aluminum oxides and spinel-group minerals. A few boast nickel above one percent by weight. One standout, named AEGIS 910A, is nearly pure silica and bears the fingerprint of hydrated silica. The diversity suggests more than one story fed these stones into the crater.<\/p>\n Location and examples of light-toned float rocks discovered at Jezero crater, Mars. (CREDIT: Communications Earth & Environment) What the rover can read from a distance<\/p>\n Because many of the rocks are small, the rover could not safely press its contact tools against them. Instead, scientists relied on remote instruments. SuperCam\u2019s laser measured elements such as aluminum, silicon and nickel, while its spectrometer searched for mineral clues. Mastcam-Z supplied context and fine color detail.<\/p>\n To interpret what they saw, researchers compared the Martian data with Earth examples. They studied deeply weathered soils near San Diego that formed about 55 million years ago. They analyzed a much older weathering profile in southern Africa known as the Hekpoort paleosol, dated to about 2.2 billion years. They also looked at kaolin formed by hot fluids in places such as Iran, Malaysia and Argentina.<\/p>\n The match that mattered most appeared between Mars and Earth\u2019s worn landscapes<\/a>, not its hot springs. The spectra of the San Diego paleosol line up closely with the target Chignik, including the telltale kaolin pattern. Chemical measures of weathering also overlap. Chignik shows strong loss of easily dissolved elements, much like the Earth soils. Another close parallel comes from the iron-poor \u201cpallid zone\u201d of the Hekpoort profile, which plots in nearly the same range as some of the Jezero rocks.<\/p>\n Titanium adds weight to that view. In long-term soil formation, titanium often becomes more concentrated because it does not move easily. Chignik and several other Martian samples contain titanium in amounts similar to the Earth soils and higher than most hydrothermal deposits. That pattern fits slow, wet weathering better than sudden heating by hot fluids.<\/p>\n Mastcam-Z landscape and multispectral images of light-toned float rocks atop the Jezero crater Margin Unit near the Hans Amundson Memorial workspace (Sol 912) demonstrating the spectral diversity of this material. (CREDIT: Communications Earth & Environment) <\/p>\n Nickel complicates the picture. High nickel often forms in deeply weathered layers on ultramafic rocks on Earth. Yet the Martian fragments lack the iron and magnesium expected in classic laterites. If nickel entered their story, it likely did so under unusual conditions.<\/p>\n Searching for their original home<\/p>\n The float rocks did not grow where Perseverance found them. Satellite data provide a few promising leads. Instruments aboard the Mars Reconnaissance Orbiter<\/a> have mapped aluminum-rich clays across the wider Jezero region. On the crater\u2019s southwest rim, light-toned outcrops and broken blocks show the same clay signals. One small patch sits just one to two kilometers from the rover\u2019s route.<\/p>\n Farther west, a long light-colored outcrop marks part of an ancient channel. Upstream, a lonely butte also shows traces of aluminum clays. Together, these sites hint at a layered crust in which aluminum clays once lay over iron and magnesium clays. Erosion, impacts or flowing water could have freed pieces and carried them into the crater.<\/p>\n One scenario ties the evidence together. Volcanic rocks near the surface slowly altered into clay-rich layers under wet conditions. Over time, burial or heat hardened parts of those layers into stone. Later, flooding, ice or impacts broke them loose and sent fragments downslope into Jezero.<\/p>\n Infrared spectra comparisons between Al-rich float rock target \u201cChignik\u201d at Jezero Crater, Mars (points 1-5, color traces, offset by\u2009+\u20090.2), and lithified E-horizon of an early Eocene (55\u2009Ma) kaolinitic paleosol, La Jolla, San Diego, CA, USA (black traces, no offset). (CREDIT: Communications Earth & Environment) Signs of a wetter Mars<\/p>\n Whether born as soils or shaped by hydrothermal systems, these rocks demand abundant water. To strip away many elements while leaving aluminum and titanium behind, water had to course through the rock in huge volumes. Such conditions fit a period early in Mars\u2019 history, when the planet was warmer and wetter than it is now.<\/p>\n If the rocks truly formed as soils, they imply rainfall lasting thousands to millions of years. Lead author Adrian Broz of Purdue University<\/a> connects that idea to Earth\u2019s tropics, where kaolinite often develops. \u201cAll life uses water,\u201d Broz said. \u201cSo when we think about the possibility of these rocks on Mars representing a rainfall-driven environment, that is a really incredible, habitable place where life could have thrived if it were ever on Mars.\u201d<\/p>\n Purdue planetary scientist Briony Horgan, a longtime planner for the rover, agrees that the stakes are high. \u201cElsewhere on Mars, rocks like these are probably some of the most important outcrops we\u2019ve seen from orbit because they are just so hard to form,\u201d she said. \u201cYou need so much water that we think these could be evidence of an ancient warmer and wetter climate where there was rain falling for millions of years.\u201d<\/p>\n